Neuromodulation, such as deep brain stimulation, spinal cord stimulation, and nerve stimulation, is becoming an increasingly preferred form of therapy for certain neurological conditions and disorders when other forms of therapy are not effective. An implantable neurological stimulation system may be used to treat conditions such as pain, movement disorders, epilepsy, depression and other medical conditions. A neurostimulation system typically includes a pulse generator and an electrical stimulation lead. A lead extension may also be used. Electrical stimulation leads have one or more electrodes, which may be positioned within or proximate to a specific location in a patient to deliver electrical energy to a target location in the patient. Some therapies involve electrical stimulation of the brain or spinal cord. Still other therapies involve electrical stimulation of other sites in the patient.
As one example, deep brain stimulation (DBS) involves delivery of electrical stimulation to nerve structures in specific areas of the brain to either excite or inhibit cell activity. A stimulation lead is typically implanted at a desired location within the brain with relative precision using magnetic resonance (MR) imaging techniques (or other imaging techniques) and stereotactic guidance. DBS can be effective in the management of, for example, chronic pain, movement disorders such as Parkinson's disease and essential tremor, epilepsy, and psychiatric disorders such as depression and obsessive-compulsive disorder.
Precise placement of the stimulation lead within the brain or other neural structure, such as the spinal cord or a nerve is important. In some applications, it is desirable to position the stimulation lead to deliver stimulation to a very small target site without stimulating adjacent neural tissue. If stimulation is not delivered with precision to a desired target site, adjoining areas may also be stimulated, which may lead to undesirable side effects.
U.S. Pat. No. 7,668,601 to Hegland et al. describes a medical lead having at least one segmented row of electrodes as well as at least one ring electrode. A preferred embodiment includes two ring electrodes and two rows of segmented electrodes, with each row of segmented electrodes including three or four electrodes each. The ring electrode is defined as extending substantially around the entire periphery of the lead body, and the segmented electrodes are defined as extending around only a portion of the entire periphery. Hegland emphasizes that the ring electrode may act as a fall-back for stimulation if the rows of segmented electrodes are not positioned proximate to the physiologically appropriate tissue for stimulation (col 3, lines 27-30).
U.S. Pat. No. 6,510,347 to Borkan describes a stimulation catheter having in-line directional electrodes. The directional electrodes are described as extending 30 to 270 degrees around the circumference of the sheath. Borkan describes that a directional electrode is preferred for spinal cord stimulation to provide a more localized stimulation region and reduce power requirements of the neuromodulation system. In a preferred embodiment Borkan describes three in-line electrodes, each extending 270 degrees.
Current electrical leads used in neuromodulation, do not provide a uniform longitudinal distribution of charge while also allowing for directional stimulation with large electrode surface area. A non-uniform longitudinal distribution of charge can make it difficult to predict the electrical field generated by selected electrodes. Further, although band electrodes are unlikely to become unintentionally detached from the periphery of the lead body since they encircle the lead body, directional electrodes do not extend around the entire periphery of the lead. Therefore directional electrodes, also known as partial or segmented electrodes, can possibly detach from the lead body, especially when being passed through a guide cannula during the implant procedure. However, an electrode with retention features that extend inward to the lead axis may require the lead diameter to be increased in order to accommodate features internal to the lead body, such as electrical conductors and/or a stylet lumen.